1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device capable of providing a wide viewing angle and high-speed response.
2. Description of the Related Art
Currently, a liquid crystal display panel utilizing characteristics such as lightness, thinness, and low power consumption is used as a display for use in television sets, personal computers and car navigation systems.
A twisted nematic (TN) type liquid crystal display panel widely utilized as this liquid crystal display panel is configured such that a liquid crystal material having optically positive refractive anisotropy is set to a twisted alignment of substantially 90° between glass substrates opposed to each other, and optical rotary power of incident light is adjusted by controlling the twisted alignment. Although this TN-type liquid crystal display panel can be comparatively easily manufactured, its viewing angle is narrow, and its response speed is low. Thus, this panel has been unsuitable to display a moving image such as a television image, in particular.
On the other hand, an optically compensated birefringence (OCB) type liquid crystal display panel attracts attention as a liquid crystal display panel which improves a viewing angle and a response speed. The OCB-type liquid crystal display panel is formed using a liquid crystal material sealed between the opposed glass substrates and capable of providing a bend alignment. The response speed is improved by one digit as compared with the TN-type liquid crystal display panel. Further, there is an advantage that the viewing angle is wide because optically self compensation is made from the alignment state of the liquid crystal material.
In the OCB-type liquid crystal display panel, as shown in (a) of FIG. 6, a liquid crystal layer having liquid crystal molecules 65 is disposed between a pixel electrode 62 disposed on a glass based array substrate 61 and an counter electrode 64 disposed similarly on a glass based counter substrate 63 which is opposed to the array substrate 61. The liquid crystal molecules 65 of the liquid crystal layer are set to a splay alignment when no voltage is applied. Thus, a high voltage of the order of some tens of voltages is applied between the pixel electrode 62 and the counter electrode 64 upon supply of power so as to transfer the liquid crystal molecules 65 from the splay alignment which is a non-display state to the bend alignment which is a display state.
To reliably transfer the alignment state upon high voltage application, voltages opposite in polarity are applied to adjacent horizontal lines of the pixels to create a nucleus by a laterally twisted potential difference between the adjacent pixel electrode 62 and transfer pixel electrode. The alignment state is transferred around the nucleus. Such an operation is carried out for substantially one second, whereby the splay alignment is transferred to the bend alignment. Further, a potential difference between the pixel electrode 62 and the counter electrode 64 is equalized, thereby temporarily eliminating an undesired record.
After the liquid crystal molecules 65 have been thus transferred to the bend alignment, a voltage exceeding a low OFF voltage, at which the liquid crystal molecules 65 are maintained in the bend alignment as shown in (b) of FIG. 6, is applied from a drive power supply 66 during operation. The OFF voltage or an ON voltage which is higher than the OFF voltage is applicable from the drive power supply 66 as shown in (c) of FIG. 6. Thus, the drive voltage between the electrodes 62 and 64 changes in the range of the OFF voltage to the ON voltage. Consequently, the liquid crystal molecules 65 are transferred between the bend alignment shown in (b) of FIG. 6 and the bend alignment shown in (c) of FIG. 6 to change a retardation value of the liquid crystal layer, thereby controlling transmittance.
In the case where an OCB-type liquid crystal display panel is used for displaying an image, birefringence is controlled in association with polarizing plates. The liquid crystal panel is driven by a driver circuit such that light is shielded (for a black display) upon application of a high voltage and is transmitted (for a white display) upon application of a low voltage, for example.
The driver circuit includes a scanning line driver circuit 67 which is formed integrally on the array substrate 61 as shown in FIG. 7 and from which a plurality of scanning lines Y1 to Yn extend in a row direction, and a signal line driver circuit (not shown) from which a plurality of signal lines X1 to Xm extend in a column direction to intersect the scanning lines Y1 to Yn.
The signal lines X1 to Xm are divided into odd numbered signal lines X1, X3, . . . and even numbered signal lines X2, X4, . . . , and drain-source paths of thin film transistors (TFTs) 68-1, 68-2, . . . 68-m′ (m′=2 m) configured as a pair of selector switches on an even number and odd number basis are connected to the respective signal lines X1 to Xm in parallel with each other. Among them, gates of TFTs 68-1, 68-3, . . . of an odd numbered set is connected to a terminal 69 to which a first selection signal is supplied, and gates of TFTs 68-2, 68-4, . . . of an even numbered set is connected to a terminal 70 to which a second selection signal is supplied, so that a video signal supplied to each of terminals 71, 72 is selected by the corresponding selection signal.
Switching thin film transistors (TFTs) 73 are disposed at intersections between the scanning lines Y and the signal lines X in which the drain-source paths of the TFTs 68-1 to 68-m′ are inserted. Each TFT 73 has a gate connected to one of the scanning lines Y1 to Yn, and a drain-source path connected at one end to one of the signal lines X. The other end of the drain-source path of the TFT 73 is connected to a liquid crystal capacitance element 74, and is connected to one end of a storage capacitance element 75. The other end of the storage capacitance element 75 is connected to a terminal 76 via a capacitance line Cs, and a storage capacitance voltage is applied from the terminal 76.
In addition, a vertical scanning clock signal and a vertical start signal are supplied to the scanning line driver circuit 67 via a terminal 77 and a terminal 78, respectively.
With such a configuration, a gate pulse from the scanning line driver circuit 67 is sequentially supplied to the scanning lines Y1 to Yn by line-at-a-time driving method, and TFTs 73 on one scanning line X are turned on simultaneously. In synchronism with this scanning, video signals from the signal line driver circuit are supplied via the terminals 71, 72 and the TFTs 68-1 to 68-m′ to the TFTs 73, to store a signal charge in each liquid crystal capacitance element 74 and the corresponding storage capacitance element 75 through the drain-source path of the corresponding TFT 73. The signal charge is held until a next scanning period has been established. Consequently, the liquid crystal capacitance elements 74 of all pixels connected to the scanning lines X are activated to display an image, the storage capacitance elements 75 are driven by a storage capacitance voltage which is applied by grounding the terminal 76 or by supplying a gate pulse in a reverse phase and supplied to the terminal 76.
In such a liquid crystal display panel, for example, in a first half of one horizontal scanning period (1H), a signal voltage having positive polarity (+) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected via the TFT 68-1 for the signal line X1, and a signal voltage having negative polarity (−) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected to the TFT 68-4 for the signal line X2, respectively, as shown in (a) of FIG. 8.
In a latter half of 1H, a signal voltage having negative polarity (−) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected via the TFT 68-2 for the signal line X2, a signal voltage having positive polarity (+) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected via the TFT 68-3 for the signal line X1.
In addition, in a next frame, in a first half of 1H, a signal voltage having negative polarity (−) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected to via the TFT 68-1 for the signal line X1, and a signal voltage having positive polarity (+) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected via the TFT 68-4 for the signal line X2, respectively, as shown in (b) of FIG. 9.
In a latter half of 1H, a signal voltage having positive polarity (+) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected via the TFT 68-2 for the signal line X2, and a signal voltage having negative polarity (−) with respect to a voltage of the counter electrode 64 is written into the pixel electrode 62 connected via the TFT 68-3 for the signal line X1. In this manner, frame inversion driving and dot inversion driving are carried out, thereby preventing an application of an undesired direct current voltage and preventing an occurrence of flickering.
In such an OCB-type liquid crystal display panel, the alignment state can be changed from the splay alignment to the bend alignment by means of a voltage applied between the display pixel electrode 62 and the opposed electrode 64.
However, in the OCB-type liquid crystal display panel, which applies a transfer drive voltage in an initial stage of a display operation, a transfer time from the splay alignment to the bend alignment greatly depends on a temperature of the liquid crystal display panel. In other words, when the temperature of the liquid crystal display panel is high, it is required that the transfer drive voltage be applied for a longer period of time. On the other hand, when the temperature is low, it is required that the transfer drive voltage be applied for a shorter period of time.
That is, a relationship between the temperature of the liquid crystal display panel and transfer time is shown in FIG. 9 as the transfer time characteristics in which the transfer time is comparatively short when the temperature of the liquid crystal display panel is high, and is long when the temperature of the liquid crystal panel is low.
As a general heating unit, a backlight serves to heat the liquid crystal display panel. However, it is believed that, in order to reduce the time of transfer driving to the minimum, a heater 83 is provided near the liquid crystal display 80 and connected to a heating power supply 82 as a heating unit 81 for heating a liquid crystal display panel 80, as shown in FIG. 10, and a thermal sensor 84 for sensing the temperature of the liquid crystal display panel 80 is provided near the liquid crystal display panel 80.
The thermal sensor 84 is connected to a temperature detecting unit 85 to detect the temperature of the liquid crystal display panel 80. Now, assuming that power has been supplied in order to transfer the splay alignment to the bend alignment, the temperature of the liquid crystal display panel 80 is sensed at the thermal sensor 84, and the temperature is detect or measured by the temperature detecting unit 85. An initial transfer time is set by an initial transfer time setting unit 86 on the basis of the initially detected temperature, and a transfer drive unit is driven by a control signal based on this information. An output of the transfer drive unit 87 is designed so as to be supplied to the liquid crystal display panel 80 via a gate and a source driver 89 controlled by a controller 88.
With such a configuration, the temperature of the liquid crystal display panel 80 is sensed by the thermal sensor 84, and a transfer drive voltage initially set corresponding to the temperature is supplied to the liquid crystal display panel 80, thereby making the liquid crystal display panel 80 operable.
In the case where such a liquid crystal display panel is used as a display device for a television set, this display panel is used under a condition in which the ambient temperature of the flat display device ranges from about 0 to 60° C., and a backlight serves as the heating unit 81 of the liquid crystal display panel 80. Further, in the case where the flat display device is used as a display for a car navigation system, the use under a severe condition such as a very hot desert area or a very cold area is presumed. Thus, it is presumed that the external environment of the system used largely changes, and consequently, the ambient temperature of the liquid crystal display panel significantly changes from below 0° C. to about 80° C. The use under an environment condition which is severer than the use in room such as a television set is unavoidable. Therefore, the heater 83 for use in heating is often used in addition to the backlight, thus making it necessary to set the operating condition of the liquid crystal panel to a use condition adapted to the external environment. In particular, in a low temperature state such as −20° C. or less, a large amount of time has been required for heating until the liquid crystal display panel 80 reaches within the range of the predetermined applicable operating temperatures, and a large amount of time such as several tens of seconds is required at the time of a low temperature as the degree of thermal effect that the transfer time affects on a temperature.
In the case where such an OCB-type liquid crystal display panel is used as the display device, an initial temperature of the liquid crystal display panel 80 is detected using the thermal sensor 84 and the temperature detecting unit 85 when the device is initially powered on. After the heating time has been set according to the initial temperature, the liquid crystal display panel 80 has been heated by the heater 83 as required.
However, it is necessary to sufficiently heat the liquid crystal display panel 80 in order to reliably carry out the initial transfer operation of the OCB-type liquid crystal display panel 80. It is unknown what degree of time is required according to a temperature difference of the liquid crystal display panel 80 to efficiently obtain a predetermined temperature for starting transfer drive. In other words, it is unknown whether or not a total of the heating time and transfer time can be minimized. Since it is necessary to provide a sufficient time margin to establish a state suitable for the initial transfer, there has occurred a problem that a large amount of time is required before start of initial transfer driving.